Delineation of Shallow Aquifer Systems and Groundwater Quality Assessment in Anaocha Local Government Area, Anambra State

Groundwater is the principal domestic water source for communities in Anaocha Local Government Area, yet the hydrogeological framework governing aquifer occurrence and water quality has not been systematically defined using modern geophysical approaches. This study delineates shallow aquifer systems and assesses groundwater quality in Anaocha LGA using vertical electrical sounding and physicochemical water quality analysis. Fifty VES stations were established on a grid pattern across the LGA using the Schlumberger array with maximum half-electrode spacing of 250 m. IPI2Win software was used to invert apparent resistivity data, producing layer models comprising three to five geoelectric units. Aquifer units are identified in the depth range 15 to 75 m within sandy sequences of the Ogwashi-Asaba Formation and the overlying Benin Formation. Resistivity values of the aquifer layers range from 40 to 280 ohm-m, suggesting predominantly freshwater-saturated coarse-grained sand. Isoresistivity and isopach maps define a major groundwater accumulation zone in the northwestern sector of the LGA where the Benin Formation attains maximum thickness. Physicochemical analyses of water samples from 18 wells show that pH, total dissolved solids, nitrate, and coliform bacteria concentrations exceed WHO guideline values in 6 of the 18 sampled wells, primarily in areas of high agricultural intensity. Protective capacity maps indicate low to intermediate aquifer protection in the affected areas. The integration of geophysical and water quality data provides a practical tool for targeted groundwater resource management in Anaocha LGA.

Keywords: vertical electrical sounding, Benin Formation, aquifer delineation, water quality, Anaocha